A variable capacitor is a capacitor whose capacitance may be altered, for example, due to a control voltage. Also referred to as a varactor, a variable capacitor may be used in any of various applications where there is a desire to adjust a capacitance, such as in inductor-capacitor (LC) circuits to set the resonance frequency of an oscillator (e.g., radio frequency channel tuning) or as a variable reactance (e.g., for impedance matching in antenna tuners).
A voltage-controlled oscillator (VCO) is an example circuit that may use a varactor in which the thickness of a depletion region formed in a p-n junction diode is varied by changing a bias voltage to alter the junction capacitance. Any junction diode exhibits this effect (including p-n junctions in transistors), but devices used as variable capacitance diodes are designed with a large junction area and a doping profile specifically chosen to improve the device performance, such as quality factor and tuning range.
More recently, variable semiconductor capacitor devices have been developed. These devices may also be referred to as transcap (TC) devices. The structure of these devices provides a variable semiconductor capacitor with a metal-oxide semiconductor (MOS)-compatible structure suitable for integrated circuits, which has at least three terminals, one of which is used to modulate the capacitance value between the other two terminals of the device, by increasing or decreasing its DC voltage with respect to one of the main terminals of the device.